The use of more than one piston in a cylinder, to provide a position memory for a second piston, by using a separate hydraulic circuit, is well known. The memory piston is typically called a "stop piston", which may or may not be permitted to move in the same part of a cylinder bore as the main working piston. Such pistons have been used to give a hydraulic cylinder rod three discrete stop positions, for power shifting of a conventional automotive transmission, and have been used to give read heads in disk-type memory systems for use with digital computers sixteen discrete positions, in response to binary-coded hydraulic inputs. Such structures have also been used in the field of agriculture, to control the lower position of agricultural ground-working tools, when they are returned to lowered positions after having been raised, such as disclosed in U.S. Pat. No. 2,596,471, issued to Densmore et al on May 13, 1952, and for an impact dampening and power lift mechanism for an outboard propulsion mechanism for a small boat, as disclosed in U.S. Pat. No. 3,434,449, issued to North on Mar. 25, 1969.
Such devices share a common deficiency, in that, the stop piston being impervious to the working fluid of a hydraulic cylinder, complicated arrangements are needed to provide working fluid under pressure to both surfaces of a stop piston, as well as to both surfaces of a working piston.
A hydraulic system may be used with a propulsion unit for a small boat to tilt the propeller and its associated driving members, out of the water, for maintenance, or for running the boat onto a shore, without damage to the propulsion unit. Such hydraulic systems also provide for the adjustment of the position of the propeller of the like with respect to the center line of the boat, so that the angle of thrust may be varied for best performance with varying loads, and under varying wave conditions. Such a system must keep the thrust of the propeller from changing the preset trim angle when it is pushing the boat forward in the water, and must also keep the propeller from changing the trim angle and pulling itself out of the water when operating in a reverse direction. This particular function is known as reverse locking. However, such a drive unit must not be held in a position unyeildingly, since it is desirable that the propulsion unit swing upward, and out of the way of submerged obstacles and the like, should a boat run over such an obstacle in operation. After being deflected by such an obstacle, it is desirable that the propulsion unit return to its original position, without further involvement by the operator of the boat. In short, such a system should also provide pressure relief and shock damping functions. These functions have been accomplished in numerous ways. For example, separate cylinders, have been used for trimming and tilting functions. Reverse locking functions have been accomplished by manual valves, pilot-operated valves, valves operated by a gear selector quadrant lever, pressure relief valves, and spring-loaded latches, with either separate small unlatching actuators or increased hydraulic pressure used to break the latches free to intentionally tilt the propulsion unit upward. Pressure relief valves and spring-loaded latches have been used to allow the propulsion unit to swing upward when contacting a submerged obstacle, and thereafter return to a position that was determined by a bolt movable to one of a number of holes, or by the end of an adjustable rod controlled by a wire from the operators' station of the boat, or the separate trim cylinder, or an auxiliary stop piston in a combined trim and tilt cylinder.
The instant invention overcomes numerous disadvantages and deficiencies of such prior structure.